1. Field of the Invention
The invention relates to a silica structure and a method of producing the same, and a heat insulating material. More specifically, the invention relates to a silica structure that has high strength, low bulk density, and low heat conductivity, and a method of producing the same, and a heat insulating material including the same.
2. Description of the Related Art
Because pores are formed inside porous materials, the porous materials have low bulk density and low heat conductivity, as compared to dense materials. The porous materials also have high heat resistance, and high corrosion resistance, depending on the qualities of the materials. Therefore, the porous materials are used as a heat insulating material, an acoustic absorption material, a filter, a weight-saving material, an adsorbing material, a catalyst support, and a separation membrane support.
The porous materials and methods of producing the porous materials have been proposed. For example, each of Published Japanese Translation of PCT application No. 10-509941, European Patent Application Publication No. 0171722, and J. Mater. Res. Vol. 8, No. 7, pp. 1736-1741 describes silica aerogel in which solid silica particles with diameters equal to or smaller than 100 nm are connected to each other in a mesh manner. Also, in http://www.taiheiyo-cement.co.jp/cement/es, a hollow particle, which has a shell made of ceramic containing silica as a main component, is described. In this site, a relatively large hollow particle with a diameter equal to or larger than 20 μm is described. The bulk density of the hollow particles is 0.25 to 0.42 g cm3, and the compressive strength of the hollow particles (when 40% remains) is 70 N/mm2, and the heat conductivity of the hollow particles is 0.1 W/m ° C. Further, Japanese Patent Application Publication No. 2004-123412 (JP-A-2004-123412) describes a method of producing a small spherical zeolite formed body, using alumina sol as a binder. Japanese Patent Application Publication No. 62-297211 (JP-A-62-297211) describes a method of producing a zeolite porous material using alumina sol as a binder.
The porous material, in which silica aerogel is used, contains a large amount of voids between the particles. Therefore, it is possible to decrease the bulk density of the porous material. However, the strength of the porous material is decreased in accordance with the decrease in the bulk density. In order to obtain sufficient strength, it is necessary to improve a filling rate. However, in this case, the bulk density of the porous material is increased, and heat conductivity of the porous material is increased. A hollow particle that includes a shell made of ceramic, such as a foamed glass bead, has strength due to the shell. Therefore, even when the diameter of the hollow particle is decreased, the shell needs to have a certain thickness to maintain the strength. However, if the diameter of the particle is decreased while maintaining the thickness of the shell at the certain thickness, the proportion of the volume of the shell to the volume of the hollow particle is increased. When using the hollow particle with a relatively large diameter as described in http://www.taiheiyo-cement.co.jp/cement/es, it is possible to decrease the density of the particle and the heat conductivity of the formed body to low values. However, as the diameter of the hollow particle is decreased, the density of the particle is increased, and as a result, the bulk density of the porous material is increased, and the heat conductivity of the porous material is increased. Similarly, it is difficult to significantly decrease the bulk density and the heat conductivity of the porous material produced by connecting zeolite particles with diameters equal to or smaller than 1 μm using alumina sol, while maintaining the strength.